1. Field of the Invention
The invention relates to a magneto-thermal device for causing a safety switch to open when the current which flows therethrough presents either an excessive instantaneous current or a smaller current but of too long a duration; this device comprises a bimetallic strip, a magnetizable yoke with which are associated: on the one hand, a screwed adjustable core which is surrounded by a coil and, on the other hand, a plate angularly movable about one end of the yoke for cooperating with one end of said core placed substantially at the same level and against a return spring, and locking means associated with said core for providing the adjustment position.
Such devices are widely used in apparatus, generally called thermal relays, which are intended for monitoring the currents flowing in a load, particularly in an electric motor, for interrupting its power supply when the current which it consumes takes on abnormal values.
2. Description of the Prior Art
In devices whose construction corresponds to the one mentioned above, it is necessary to adjust the core so that the intensity of the current which flows in the coil causes the plate to be attracted when a well-defined current threshold is reached; this current threshold which is determined as a function of the characteristics proper to each motor, causes tripping, as a function of the different geometrical parameters of the coil, of the magnetic circuit and of the return spring, which are subject to slight variations during mass production; furthermore, an adjustment carried out in the factory so that the finished apparatus has the desired protection properties, must be kept during fitting and thereafter when either mechanical or thermal stresses or ageing of the materials of the apparatus or of its case appear.
Among the adjustments whose importance is preponderant are those which influence the dimensions of the air-gaps which exist between the end of the core, the plate and the end of the yoke; modification of the first of these air-gaps must especially not influence the second one. In some known devices, adjustment of the respective position of the active end of the core and of the plate is provided by moving an opposite threaded end into a threaded opening in the yoke, whereas locking of the chosen adjustment is provided by means of a locknut which is engaged against this threaded end. Such a known device has the disadvantage of a drive being possibly communicated to the core at the time when a locking torque is communicated to the nut; when the pitch of the thread is fine, such a drive is small, but, even in this case, it is not excluded that the locking forces may cause a slight lateral movement of the active end and modify the leak air-gap which exists between it and the end of the yoke.
The invention consequently provides a device conforming to the one whose construction is mentioned above but in which measures will be taken so that the means for locking the core do not modify the lateral position of the active end of the core and ensure permanence of the adjustment effected.